Cholesteric liquid crystal stabilizers for detector elements

ABSTRACT

A sensing device is made by protectively packaging a cholesteric liquid crystalline material containing from about 0.3 to 7 weight percent of an ultraviolet radiation stabilizer consisting of either aromatic azo or aromatic azoxy compounds, in a clear compatible protective polymeric material.

United States Patent.

Dixon et al.

[54] CHOLESTERIC LIQUID CRYSTAL STABILIZERS FOR DETECTOR ELEMENTSInventors: George D. Dixon, Monroeville; Luciano C. Scala, Murrysville,both of Pa.

Assignee: Westinghouse Electric Corporation, Pittsburgh, Pa.

Filed: Feb. 16, 1970 Appl. No.: 11,449

U.S. Cl 23/253 TP, 23/230 LC, 252/408,

350/ l 60 Int. Cl. ..C09k l/00, GOln 31/22, G02f 1/00 Field of Search..23/253 TP, 230 LC; 252/408 [151 3,656,909 Apr. 18, 1972 [56]References Cited FOREIGN PATENTS OR APPLICATIONS 1,161,039 8/1969 GreatBritain ..23/230 LC Primary Examiner-Morris O. Wolk AssistantExaminer-R. M. Reese Att0rneyA. T. Stratton and Alex Mich, Jr.

[5 7] ABSTRACT A sensing device is made by protectively packaging acholesteric liquid crystalline material containing from about 0.3 to 7weight percent of an ultraviolet radiation stabilizer consisting ofeither aromatic azo or aromatic azoxy compounds, in a clear compatibleprotective polymeric material.

12 Claims, 6 Drawing Figures PHOTOMULTIPLIER I POWER SUPPLY TOPHOTOMULTIPLIER o 0 NULL DETECTOR QEQ BLACK COLOR B PAINT FILTER [1 LENSI [I POWERTgUPPLY POTENTIOMETER I HELIUM LAMP l \p \L COLLIMATING HELIUMTUBE LIQUID CRYSTAL LAMP FILM POWER SUPPLY TH2 Tc TO RECORDER REFERENCEJUNCTION IN MELTING M X -Y RECORDER TE- THERMOELECTRIC ELEMENT THI, TH2THERMISTORS TEMPERATURE CONTROLLER AND POWER SUPPLY TO THERMOELECTICUNIT PATENTED PR 18 I912 3,656,909

SHEET 10F 3 FIG. I.

TO I PHOTOMULTIPLIER 0 0 NULL v DETECTOR QE E :11 L T COLOR B FILTER I'mQ S POTENTIOMETER HELIUM LAMP I k COLLIMATING \HELIUM TUBE LAMP t POWERSUPPLY 2\ TO c: RECORDER fi REFERENCE JUNCTION IN MELTING ICETEMPERATURE CONTROLLER RECORDER AND POWER SUPPLY To THERMOELECTIC UNITAL- ALUMINUM BLOCK cu-' COPPER BLOCK HQ 2 B WORKING BATTERY Tc-THERMOCOUPLE TE- THERMOELECTRIC ELEMENT TH|,TH2 THERMISTORS WITNESSESINVENTORS wffim 1125532925? M 1. 6% Z6 4 7M4 A I ATTORNEY PATENTEIJIPRI8 I972 SHEET 2 OF 3 FIG. 3.

I000 TIME IN HOURS O O nw O 2 I m B :A A 3 54 4 2 4 P? V HU,O O 0 Mk0 n0 O O O O J o o 0 FIG. 4.

PATENTEDAPR 18 1912 PEAK REFLECTANCE TEMPERATURE (55003)0.

PEAK RESPONSE TEMPERATURE (5500K) C.

SHEET 3 BF 3 FIG. 5.

2600 3600 TIME IN HOURS TIME IN HOURS FIG. 6.

BACKGROUND OF THE INVENTION Objects emit infrared radiation, which isinvisible to the naked eye, having an intensity which is a function ofthe temperature and the emissivity of the object. Much effort has beendirected to the problem of converting this heat image,

which is invisible to the eye, into an image which can be seen.

Thermally responsive materials and devices would have many applicationsand could be used for example, as a direct visual indication that anobject is so hot that contact with it is dangerous. Apart from thesafety aspect, visual temperature indications by direct means wouldprovide a decorative and useful agency.

Materials useful for such purposes have been taught be Fergason and Voglin US. Pat. Nos. 3,114,836 and 3,409,404. These materials arecholesteric liquid crystals.

These liquid crystals or anisotropic liquids have been used astemperature, gas, electric field and shear sensing devices in manyindustrial and medical applications. However, their use is not aswidespread as their properties would warrant because, on standing, thesesensors lose their characteristic color response, as shown both by afading of their intensities and by a shift of their maximum peaktemperatures. We found that ultraviolet radiation and airborneparticulate contaminants are mainly responsible for the degradation ofthe useful properties of detector elements made with cholesteric liquidcrystal materials.

SUMMARY OF THE INVENTION Our invention solves the prior art problems andprolongs the useful life of cholesteric liquidcrystals by protectivelypackaging them in a substantially homogeneous mixture of stabilizers ofthe azoxy and azo-type and/or solid or liquid compatible protectivepolymeric materials.

I BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of theinvention, reference may be made to the exemplary embodiments shown inthe accompanying drawings in which:

FIG. 1, shows, in elevation, an anisotropic cholesteric liquid crystalon a substrate in homogeneous mixture with a radiation absorber and aprotective polymeric material;

FIG. 2, shows, in schematic form, the thermooptical apparatus used tomeasure the useful life of the liquid crystal containing structures inthe examples;

FIG. 3, is a graph showing the effect of ultraviolet radiation on theresponse of a standard (OCC-CN-CC) liquid crystal mixture not containingan ultraviolet stabilizer additive;

FIG. 4, is a graph showing the effect of ultraviolet radiation on theresponse of a standard (OCC-CNCC) liquid crystal mixture with stabilizerA[( 1 ,2,4 percent by weight of 4-(phenyl azo) phenol], B [(4,5 ,6percent by weight of 4-(4'-ethoxy phenyl azo) phenyl l-undecenoate)] andC[(4,5,6 percent by weight of 4-(4'-ethoxy phenyl azo) phenylhexanoate)];

FIG. 5, is a graph showing the effect of ultraviolet radiation on theresponse of a standard (OCC-CN-CC) liquid crystal mixture withstabilizer D[l,2,4 percent by weight of azoxydianisole)] and additiveF[(4,6 percent by weight of cholesteryl 4-(phenyl azo) phenyl carbonate)1; and

FIG. 6, is a graph showing the efiect of ultraviolet radiation on theresponse of a standard (OCC-CN-CC) liquid crystal mixture withstabilizer E [(4,5,6 percent by weight of cholesteryl 4-(phenyl azo)benzoate)] and additive G 1,3,4 percent by weight of benzophenone)].

DESCRIPTION OF THE PREFERRED EMBODIMENTS Liquid crystalline materialshave properties that are intermediate between those of a true liquid andthose of a true crystal, since they have an ordered structure while alsoexhibiting fluidity. Liquid'crystalline materials are also referred toas materials in the mesomorphic state. Liquid crystalline materials areknown and are characterized or identified by one of three phases orstructures. One is the smectic structure which is characterized by itsmolecules being arranged in layers with the long axis approximatelynormal to the plane of the layers. The second is the nematic structure,which is characterized by thread-like molecules that tend to be andremain in nearly parallel orientation to each other but nor arranged indiscrete layers. The third is known as the cholesteric phase, themolecular configuration of which is a modification of the nematicstructure. The present invention is concerned with materials whichexhibit a cholesteric liquid crystalline phase and which are useful asdetector elements giving visible response to a wide variety of externalstimuli such as electric and magnetic fields, shear, pressure and heat.

The cholesteric phase has certain characteristics which are markedlydifferent from either the smectic or the nematic phase. Thecharacteristic properties of the cholesteric structure may be summarizedas follows: 1) It is optically negative, while smectic and nematicstructures are optically positive. (2) The structure is opticallyactive. It shows strong optical rotary power. (3) When illuminated withwhite light, the most striking property of the cholesteric structure isthat it scatters light selectively to give vivid colors. A cholestericmaterial exhibits a scattering peak having a bandwidth of about 200Angstroms that occurs in or between the infrared and ultravioletportions of the spectrum. (4) In the cholesteric structure, one circularpolar component of the incident beam is completely unafiected. For thedextro cholesteric structure, it is only the circular polarized beamwith counterclockwise rotating electric vector which is reflected. (Thesign of rotation refers to an observer who looks in the direction of theincident light.) Levo cholesteric structures have the reverse effect.(5) When circular polarized light is scattered from these materials, thesense of polarization is unchanged. In ordinary materials, the sense ofcircular polarization is reversed. (6) The mean wave length of thereflection band depends upon the angle of incidence of the beam. Therelationship can be roughly approximated by the Bragg diffractionequation for a birefringent material. These enumerated propertieseffectively define cholesteric liquid crystals. For further generalinformation on this broad class of materials, reference should be madeto the article by G. H. Brown and W. G. Shaw in Chemical Reviews, vol.57, No. 6, Dec. 1957, p. 1,049, entitled The Mesomorphic State LiquidCrystals.

Some examples of suitable cholesteric liquid crystalline materials whichcan be used alone or in mixtures as electric and magnetic field, shearpressure and heat sensing compounds in this invention include mixedesters of cholesterol and inorganic acids such as cholesteryl chloride,cholesteryl bromide, cholesteryl nitrate, etc.; organic esters ofcholesterol such as cholesteryl crotonate, cholesteryl nonanoate,cholesteryl formate, cholesteryl acetate, cholesteryl propionate,cholesteryl valerate, cholesteryl hexanoate, cholesteryl docosonoate,cholesteryl chlorofonnate, cholesteryl linolate, cholesteryl linolenate,cholesteryl oleate, cholesteryl erucate, cholesteryl butyrate,cholesteryl caprate, cholesteryl laurate, cholesteryl myristate, etc.;ethers of cholesterol such cholesteryl decyl ether, cholesteryl laurylether, cholesteryl dodecyl ether, etc.; carbonates and carbamates ofcholesterol such as cholesteryl decyl carbonate, cholesteryl methylcarbonate, cholesteryl ethyl carbonate, cholesteryl butyl carbonate,cholesteryl docosonyl carbonate, cholesteryl cetyl carbonate, oleylcholesteryl carbonate, cholesteryl heptyl carbamates, etc.; alkyl amidesand aliphatic secondary amines derived from 3-beta-aminodelta-S-cholestene, and the corresponding esters noted above ofcholesterol, etc.

The alkyl portion of the above compounds comprises at least one compoundselected from the group consisting of saturated and unsaturated fattyacids and alcohols having from one to 27 carbon atoms per molecule, theunsaturated members having from 1 to 6 olefinic groups per chain. Arylsubstituents generally comprise a single benzene ring that may have oneor more lower alkyl groups attached thereto.

The foregoing compounds exhibit a cholesteric liquid crystalline stateover a given range of temperature. These temperature ranges in instancesare small, and large in other instances for the list of materials given.The temperatures may be as low as about minus 20 C. to as high as about250 C. The determination of the range for each material is easily madeby heating the compound (or mixture) and noting the appearance ofturbidity or possibly a color. After a further rise and at a definitetemperature, the material clears to a true liquid and is no longer inthe cholesteric liquid state. Upon cooling from the true liquid state,the action is reversed, though supercooling may depress the first notedtemperature. The consistency of the various materials may range from athick paste to a freely flowing liquid, while in the liquid crystallinestate. The materials can be used in this state. Some are dissolved in asuitable solvent, for example, chloroform, ether, benzene, petroleumether, petroleum naptha, carbon tetrachloride, common saturatedhydrocarbon mixtures such as kerosene, and carbon disulfide, or othercommon organic solvents, and then poured onto a film or substrate fromwhich the solvent evaporates. These films frequently can be separatedfrom the support and located where desired.

Accordingly, each of the liquid crystal materials used in this inventionhas a characteristic temperature range at which it is to be used. Forexample, cholesteryl caprate exhibits the phase from 82 to 89 C; forcholesteryl chloride, the range is 95 to 97 C.; for cholesterylcinnamate, the range is about 161 to 197 C.; for oleyl cholesterylcarbonate, the range is about 18 to 33 C.; for cholesteryl nonanoate, itis 79 to 90 C.; for cholesteryl arachidonate, the liquid crystallinephase occurs below C.; for cholesteryl p-nitro benzoate, the liquidcrystalline phase occurs in the range from about 189 to 250 C. at whichlatter temperature it decomposes. Many mixtures of compounds forming thecholesteric liquid crystalline state form cholesteric liquid crystalphases at room temperature and below. Considering the compounds andmixtures as a whole, detectors are thus now available to operate forranges of to 100 C. at any center temperature of +10 C. to in excess of150 C. As is apparent, in using these materials in this invention, thematerials will be used at the appropriate temperature to secure thecholesteric liquid crystalline phase.

Many of the foregoing compounds are available commercially, havingsubstantial commercial uses. Others are disclosed in the literature towhich reference can be made for details of preparations as well asgeneral properties. Some methods of synthesis found to be especiallyuseful are as follows: Cholesteric liquid crystals comprising carboxylicacid esters of cholesterol can be prepared by heating cholesterol and acarboxylic acid to the boiling point of the acid, or, in the case ofhigh molecular weight acids, to about 200 C. After thorough reaction,the mixture is cooled to handling temperature. Cholesterol andcarboxylic acids can also be made to react in a benzene solution or inother volatile hydrocarbon solvents upon the addition of a catalyst, forexample, paratoluene sulfonic acid. Another useful method comprisesreaction of an acyl halide with cholesterol in the presence of asuitable proton acceptor, for example pyridine or analogous compounds.This latter reaction can be performed in the presence of a solvent ifdesired though none is needed.

Using the latter process, cholesterol and pyridine can be dissolved inequal amounts in benzene. The acyl chloride being used is also dissolvedin a similar amount of benzene and in a like molar quantity. Then thislatter solution is added dropwise to the cholesterol-pyridine-benzenesolution. The reaction proceeds spontaneously, usually with theevolution of heat and the formation of a fine white precipitate ofpyridine hydrochloride. After complete addition of the acyl chloride,the mixture is refluxed for about 1 hour to insure complete reaction.Then the mixture is cooled to room temperature, the precipitate isfiltered, washed with benzene and discarded.

The filtrate and washings are then treated with a lower alkyl alcohol,for example methyl or ethyl alcohol. Crystallization is promoted by slowaddition of the alcohol while constantly stirring. Recrystallization canbe practiced to obtain the pure product. Cholesteryl alkyl or arylcarbonates can be readily made by first reacting phosgene withcholesterol, and reacting the product with the appropriate alcohol, inthe presence of a proton acceptor, to produce the mixed ester carbonate.Suitably a solvent such as benzene is used as the reaction medium. Othersuitable methods of synthesis can be used as desired.

We found that damage to the color characteristics of cholesteric liquidcrystals exposed to artificial or natural light is in great part due tothe effect of ultraviolet irradiation, which could reduce the usefullife of liquid crystal films as a detector element to less than 2 weeks.

We found that aromatic azo compounds Ar-N NAr'and aromatic azoxycompounds having stable substitutions on one or both of the aryl (Ar orAr) rings, when added to cholesteric liquid crystalline materials,materially decreased the deleterious effects caused by ultravioletradiation. The azo and azoxy compounds useful in this invention arethose that while prolonging the useful life of liquid crystals, do notsubstantially alter their color response.

By stable substitutions on the aryl rings of aromatic azo and aromaticazoxy compounds is meant: alkyl, halo, nitro, hydroxy, alkoxy, aroxy,ether and cyano substitutes.

Azo compounds can be named in two ways. The less complicated ones arenamed as derivatives of azobenzene. Positions of substituents in therings are usually indicated by numbers, primes being used to distinguishbetween positions on the two rings. More complicated azo compounds canbe named by considering the arylazo group Ar N N as a substituent. Forfurther general information on this class of materials which are usedmostly today as dyes, reference should be made to Morrison and Boyd,Organic Chemistry, Allyn and Bacon, lnc., 2nd Ed., 1966, Chapter 24.1 1.

A variety of substituted aromatic azo and azoxy compounds were added tocholesteric liquid crystals to give sensor compositions. Among thosefound particularly suitable in increasing heat sensing life of thecompositions were:

A. 4-(phenyl azo) phenol B. 4 (4-ethoxy phenyl azo) phenylIO-undecenoate C. 4-(4-ethoxy phenyl azo) phony] hexanoate and crystalplus stabilizer), the stabilizer will crystallize out of the liquidcrystal mixture. Generally, at below about 0.5 weight percent, theeffect of the additive stabilizer in protecting the liquid crystal willnot be very noticeable.

Degradation of the useful properties of cholesteric liquid crystals isdue also to the influence of airborne particulate impurities. It wasfound that protecting a cholesteric liquid crystal material with certainclear colorless compatible polymeric materials increased the lifetime ofthe liquid crystal by preventing textural changes caused by atmosphericcontaminants. Dust particles and such alighting on the surface of theliquid crystal without such protective packaging, would produce rippledareas of non-color response surrounding the particles.

Referring now to the drawings, FIG. 1 shows a sensing device, comprisinga substrate support member upon which a detector element 11 has beendeposited. This detector element may contain liquid crystals,ultraviolet radiationstabilizers and polymeric protective material in asubstantially homogeneous gel or solid phase mixture.

Generally, the criterion for the use of any material as a supportingsubstrate member, is that it not interfere, as by reacting with theliquid crystal that is to be deposited thereon, or affecting the opticalproperties of the cholesteric liquid crystal. Typical materials thathave been used solely as a support include halogenated hydrocarbonresins (such as polytetrafluoroethylene), polyethylene terephthalate,glass, methylmethacrylate resins, polycarbonate resins and ceramlCS.

The liquid crystals in mixture with a protective polymeric material canbe applied to the substrate member in a number of ways includingpainting, casting, or application with a dropper or spatula. A quitecommon method of application is to pour a solution containing the liquidcrystal and protective polymeric material onto the support and allow thesolvent to evaporate. This solution could also contain aromatic azo andazoxy stabilizer compounds in accordance with this invention.

One method of producing the detector element 11 in FIG. 1 is casting orotherwise depositing on substrate 10 a mixture containing (1) a sensorcomposition of cholesteric liquid crystalline materials and generally anultraviolet radiation stabilizer, in a first solution and (2) chemicallycompatible protecting polymeric materials, in a second solution. Thismixture is then allowed to evaporate. The protecting polymeric solutionmust be one in which the liquid crystals are soluble. or miscible, suchas, solutions of silicone resins and low molecular weight resinouscopolymers of isobutylene and isoprene (butyl rubber). Other suitableprotective polymeric materials would include polyvinyl chloride,polystyrene, polyacrylate, polycarbonate, polyesters, polyisoprenes andlinear polyurethanes.

These are polymeric materials well known in the art and it is to beunderstood that they will be dissolved in suitable solvents in thepractice of this invention. For example: cyclohexanone andtetrahydrofuran for polyvinyl chloride; benzene, toluene, ethyl benzene,carbon tetrachloride, chloroform, and o-dichlorobenzene for polystyrene;ethyl acetate, ethylene dichloride, trichloroethylene, chloroform, andtoluene for polyacrylate; methylene chloride, chloroform,tetrahydrofuran and cis-l,2-dichloroethylene for polycarbonates;mixtures of phenol and tetrachloroethane for polyesters such aspolyethylene terephthalate; benzene for polyisoprenes such as butylrubber; and toluene for linear polyurethanes. Further reference may bemade to 1968 Modern Plastics Encyclopedia pages 84-101 for theproperties and effect of organic solvents on the above enumeratedprotective polymeric materials.

The protective packaging of the liquid crystals and ultraviolet absorbersensor compositions can also be accomplished by admixing the protectivepolymeric material in low molecular weight liquid form (averagemolecular weight of the polymer less than about 10,000). In this methodthe detector element will be cast or otherwise deposited on a substrateas a mixture containing: l) a sensor composition of cholesteric liquidcrystalline materials and generally an ultraviolet radiation stabilizer,in solution and (2) low molecular weight, liquid, chemically compatible,protecting polymeric material. Suitable low molecular weight, protectingpolymeric materials would include silicone resins, polyisoprenes,polyvinyl chloride, polystyrene, polyacrylate, polycarbonate, polyestersand linear polyurethanes.

In all cases, the resulting cast detector element, whether in gel orsolid form, will comprise a substantially homogeneous mixture ofprotective polymeric material (resin) and liquid crystalline material,with or without ultraviolet radiation absorbers. The protectivepolymeric material can comprise between about I to 20 weight percent ofthe resulting cast mixture after evaporation of solvents, with apreferred range of between about 3 to 10 weight percent. If below 1weight percent of the evaporated substantially homogeneous mixture isthe protective polymeric material, the protective effect is too smalland if above 20 weight percent of the evaporated substantiallyhomogeneous mixture is the protective polymeric material, the responsecolors are diluted. I

FIG. 2 shows the apparatus used in determining aging effects of theliquid crystals, by recording the behavior of the peak reflectancetemperature using a 5,500 A. wavelength optical filter, as a function oftime. The liquid crystal sample was tested on this apparatus, afterbeing irradiated with ultraviolet light having a wavelength of 3,660 A.,at regular time intervals to determine whether their maximum reflectancetemperatures (temperature at which the liquid crystals give their mostintense color) and color intensities had changed for a certain color(5,500 A. filter).

The He-light source illuminates the liquid crystal. The X-Y recorderidentifies the peak wavelengths of the reflected light and theirrelation to temperature. Light reflected by the liquid crystal travelsupward through a lens into a photomultiplier tube shielded from thelamp. The suitable amplified output of the photomultiplier tube is fedinto the Y-axis of the recorder. The amplitude of the Y ordinatesdepends on the photomultiplier current which in turn depends on theintensity of the reflected light reaching the photomultiplier cathodesthrough the lens. The temperature of the liquid crystal film can bevaried by means of a thermoelectric (T.E.) element which providesheating or cooling as required.

As shown in FIG. 2, an aluminum block placed between the T.E. elementand the film provides some heat storage and the location for temperaturecontrol and measurements. Three holes drilled in this block accept twotherrnistors (TH-l and Tl-l-2) and a thermocouple (T.C.). ThermistorTH-l forms part of an electrical circuit which feeds the X-axis of therecorder. The thermocouple (T.C.) connected to a handoperated highprecision potentiometer, affords high accuracy and the thermocouplereading gives a separate and individual calibration for every test runmade.

A copper heat sink and cooling arrangement are provided "for the T.E.elements, and the optical setup is enclosed in a dark box to excluderoom light.

EXAMPLE 1 The cholesteric liquid crystal ingredients obtained fromcommercial sources were found to contain, in some cases, as much as 20weight percent impurities, whose temperature and field effect responseswere different from those of the desired heat sensing material. It wasfound that the three cholesteric compounds used for this work(cholesteryl nonanoate, cholesteryl chloride and oleyl cholesterylcarbonate) could be satisfactorily purified (i.e., impurities could notbe detected by differential thermal analysis) by means of at least tworecrystallizations.

The standard cholesteric liquid crystal mixture used was: 33 parts byweight of purified oleyl cholesteryl carbonate (OCC- recrystallizedtwice from hot acetone), 40 parts by weight of cholesteryl nonanoate(CN-recrystallized twice from hot methyl alcohol), and 27 parts byweight of cholesteryl chloride (CC-recrystallized twice from hot methylalcohol).

These ingredients were thoroughly mixed and then dissolved inanalytically pure chloroform to give a 10 percent by weight solution.

This solution was poured onto 20 mil (0.020 inch) thick polyethyleneterephthalate (MYLAR) sheets stretched over a brass hoop. The chloroformsolvent was allowed to evaporate and the resulting liquid crystal filmwas evacuated at 10 mm Hg pressure to remove all traces of solvent. Theresulting cholesteric liquid crystal films (33 wt. OCC, 40 wt. CN, 27wt. CC) were about 20 microns thick.

The liquid crystal films mounted on the MYLAR sheets were placed in aPlexiglass (polymethyl methacrylate) support frame, the top of whichacted as a filter opaque to wavelengths shorter than 3,600 A., andirradiated by a Gelman-Camay Model 51402 ultraviolet light source (3,660A. Hg lamp) at a distance of 10 cm, in an atmosphere of 21 v/v percentoxygen in argon. The peak reflectance temperatures were measured atregular time intervals on the apparatus shown in FIG. 2 of the drawingsand described in the specification, to determine if the maximumreflectance temperature and intensity had changed for a certain color(5,500 A. filter) due to accelerated irradiation of the liquid crystalfilms. One hour short distance exposure to the Hg lamp was equivalent toabout from 3 to hours exposure to outdoor or normal fluorescentlighting.

The color response life of the unpackaged unstabilized liquid crystalsample dropped abruptly, and the peak reflectance temperature changeddramatically after 500 hours of exposure to the Hg lamp. After 250hours, the maximum reflectance temperature changed more than 3 C. FIG. 3shows the behavior of the peak reflectance temperatures (only 5,500 A.shown) as a function of time for the irradiated sample.

EXAMPLE 2 To the standard liquid crystal mixture of Example 1 (33 partsoleyl cholesteryl carbonate, 40 parts cholesteryl nonanoate, 27 partscholesteryl chloride) was added from 1 to 6 weight percent of thefollowing ultraviolet absorbing stabilizers:

A. 4-(phenyl azo) phenol: l, 2, 4 and 5 weight percent;

B. 4-(4'-ethoxy phenyl azo) phenyl IO-undecenoate: 4, 5,

and 6 weight percent;

C. 4-(4'-ethoxy phenyl azo) phenyl hexanoate: 4, 5 and 6 weight percent;

D. Azoxydianisole: l, 2, 4 and 5 weight percent;

E. Cholesteryl 4-(phenyl azo) benzoate: 4, 5 and 6 weight percent;

F. Cholesteryl 4-(phenyl azo) phenyl carbonate: 4, 5 and 6 weightpercent; and

G. Benzophenone: l, 3 and 5 weight percent.

After addition and thorough mixing of the stabilizers, thestabilizer-liquid crystal materials were dissolved in analytically purechloroform solvent to give 10 percent by weight solutions. Thesesolutions were poured on mil thick MYLAR sheets stretched over a brasshoop. The solvent was allowed to evaporate and the resulting film wasevacuated at 10 mm Hg pressure to remove all traces of solvent. Theresulting liquid crystal films were about 20 microns thick.

The stabilized cholesteric liquid crystal films mounted on the MYLARsheets were placed in a Plexiglass support frame, the top of which actedas a filter opaque to wavelengths shorter than 3,600 A., and irradiatedby a Gelman-Camay Model 51402 Ultraviolet light source (3,660 A. Hglamp) at a distance of 10 cm, in an atmosphere of 21 v/v percent oxygenin argon. As in Example I, the peak reflectance temperatures weremeasured at regular time intervals on the apparatus shown in FIG. 2 ofthe drawings and described in the specificatron.

The following Table 1 gives the period of time over which TABLE 1 Colorresponse life: hours of exposure, 3660 A. Ultraviolet radiation at 10cm., for stabilizer, percent of sample [Sample: (OCC, CN,CC)+stabilizer.]

Hours Standard plus A 4,200+ 4, 200+ 4,200+ 4, 200+ 1 No color after 600hours. 2 No color after 1,000 hrs. 8 No color after 100 hrs.

FIGS. 4, 5 and 6 show the behavior of the peak reflective temperatures(only 5,500 A. shown) as a function of time for the irradiatedstabilized samples.

FIG. 3 shows the extreme sensitivity of liquid crystals not containingstabilizers to ultraviolet radiation; in fact, the useful life of theunprotected system is of the order of 250 hours.

Stabilizers A [(4-(phenyl azo) phenol)], B [(4-(4-ethoxy phenyl azo)phenyl l0-undecenoate)], and C [(4-(4-ethoxy phenyl azo) phenylhexanoate)], as shown in Table l and FIG. 4, produce an enormous (almostinfinite increase in the useful life of the liquid crystal system underthe same accelerated aging conditions as the standard withoutstabilizers, and are relatively concentration independent. Theseadditives are extremely efiective in shielding the liquid crystalmixture from the efiect of ultraviolet irradiation.

Stabilizers D [(azoxy dianisole)] and E [(cholesteryl 4- (phenyl azo)benzoate)], as shown in Table l and FIGS. 5 and 6, increase the usefullife of the liquid crystal system to about 1,000 hours (a minimum ofabout a 400 percent increase) under the same accelerated agingconditions as the standard without stabilizers. The effects here areconcentration dependent. These additives are very effective in shieldingthe liquid crystal mixture from the effect of ultraviolet irradiation.

Additive F [(cholesteryl 4-(phenyl azo) phenyl carbonate)], acholesteryl carbonate ester, has a useful lifetime of about the sameorder as the standard unstabilized sample of Example I, as shown in FIG.5. The cholesteryl carbonate ester moiety in additive F ill the maximumreflectance temperature changed no more than :2" to 3 C.

shown by the room temperature instability of OCC), being rapidlydegraded in the presence of 0 Benzophenone, addiis a known ultravioletradiation absorber. It is not an azo or azoxy compound and gives verypoor results, in fact, accelerating the degradation to a rate which isfaster than that exhibited by the standard unstabilized sample ofExample I as shown in FIG. 6. Benzophenone is an active producer of freeradicals and causes rapid decomposition of the liquid crystal filmpresumably due to attack on oleyl cholesteryl carbonate (OCC).

Tests were also run to determine the long term effect of the stabilizerson the regular color response of the stabilized liquid crystal mixturesin the absence of ultraviolet radiation. The stabilized liquid crystalmixtures of Table l were aged in a refrigerator at 5 C. in the absenceof ultraviolet radiation for 1,224 hours and the peak reflectancetemperatures measured (5,500 A. optical filter). We found thatstabilizers A, B, C, D and E could be safely used after about 2 monthsstorage without appreciably affecting the color response of the liquidcrystal system.

cholesteric liquid crystal films (33 wt. OCC, 40 wt. CN, 1

27 wt. CC) were about 20 microns thick.

A second sheet of MYLAR was then placed over one of the liquid crystalfilms and first MYLAR sheet to encapsulate and provide protectivepackaging for the liquid crystal. This second sheet was about 2 milsthick.

Liquid crystals in a first solution were also protectively packaged bycasting them together with either a second solution of 5 percent weightto volume poly 'y -benzyl L-glutamate (sold by Pilot Chemical Co.) ontoa MYLAR film support or a second solution of 20 percent weight to volumepoly t-butylisocyanate, and allowing the solutions to evaporate. Thecolorless polymeric protective packaging materials in the evaporated mixacted as the protective agent for the liquid crystals. The evaporatedmix was a homogeneous mixture of polymeric protective material andliquid crystals.

Similarly, portions of the standard cholesteric liquid crystal mixture,in percent chloroform solution, were also homogeneously mixed withvarious other plastic polymeric fluids and the mixture cast into filmswhich after evaporation comprised a homogeneous liquidcrystal-protective packaging combination. To 9 parts of the standardliquid crystal mixture in 10 percent chloroform solution, 1 part ofclear liquid silieone resin as a 10 percent solution in xylene solvent(sold under the trade name R-6l l Silicone by Union Carbide Corp. havinga viscosity at 25 C. of about 135 cp. and a specific gravity of about1.054), was added. Similarly, to 9 parts of the standard liquid crystalmixture in 10 percent chloroform solution, 1 part of clear liquid butylrubber as a 10 percent solution in benzene (low molecular weightcopolymer of isobutylene and isoprene sold under the trade name of LMButyl 504 by Enjay Chemical Co.), was added. Both of the clearprotecting fluids were soluble in the liquid crystal solution. Bothmixtures were thoroughly blended, cast as a film on a MYLAR substrateand allowed to evaporate. The self packaged, homogeneously encapsulated,cholesteric liquid crystal heat sensing article, containing about 10weight percent protective polymeric packaging materials was about 5 milsthick.

As a control sample, one supported cast liquid crystal film micronsthick was left unprotected.

All of the packaged liquid crystal films and the unpackaged liquidcrystal film were irradiated as in Examples 1 and 2, and the change incolor response noted using the apparatus shown in FIG. 2, and describedin the specification. Table 2 gives the test results for theprotectively packaged heat sensing articles:

TABLE 2 Effect of Protective Packaging Combination On The Initial ColorResponse of Standard Liquid Crystal System These results show that allof the systems tested except the liquid crystal system protected by polyt-butylisocyanate have promise, especially since the intensity of thecolor response was not appreciably affected by the protective packagingtreatment. Liquid crystal systems also containing 0.4 percent by weight4-(4'-ethoxy phenyl azo) phenyl hexanoate stabilizer gave similarsatisfactory results.

The samples of Table 2 were then left open to the atmosphere for severalweeks. The unprotected liquid crystal film developed craters one-halfhour after preparation and was completely degraded after 2 weeks.Initial observations of degrading liquid crystal films led one tobelieve that crater formation was associated with the presence ofairborne, solid contaminants. Microscopic examination showed that thesewere in fact dust particles or fibers in a great number of the craterswhich had formed. The larger craters (about oneeighth inch in diameter)usually had a region that no longer contained liquid crystallinematerial.

In the majority of cases where craters had formed there was a ring ofsmall, focalconic spherulites having definite boundaries inside and out.The outside of this ring made contact with the main body of the liquidcrystal which exhibited the cholesteric plane texture. Many smallcraters (less than 300 microns diameter) did not have a definite innerboundary and, occasionally, the focal-conic spherulites extended to thecenter of the disturbance. The immediate eflect of a foreign body, suchas an airborne fiber, was to cause a drastic change in the orientationof the liquid crystal surrounding the impact area. This then developedinto a crater as described.

The lifetime of the liquid crystal protected by polyethyleneterephthalate sheets was lengthened by preventing crystalline breakdownarising from atmospheric contaminants. However, crystallineincongruities in the polyethylene terephthalate surface could also causechanges in the color response of the liquid crystal film.

The other systems: liquid crystal protected by poly y -benzylL-glutamate, poly-t-butylisocyanate, silicone resin and butyl rubber,protected the liquid crystal films without any deleterious efiects andno large number of craters could be observed after a 2 week period.

We claim:

1. A composition of matter comprising a cholesteric liquid crystallinematerial and from about 0.3 to 7 weight percent of an ultravioletradiation stabilizer selected from the group con sisting of substitutedaromatic azo compounds and substituted aromatic azoxy compounds whereinthe substituents on the aryl rings of the azo and azoxy compounds areselected from the group consisting of alkyl, halo, nitro, hydroxy,alkoxy, aroxy, ether and cyano substitutions.

2. The composition of matter of claim 1 wherein the stabilizers areselected from the group consisting of 4-(phenyl azo) phenol,4-(4'-ethoxy phenyl azo) lO-undecenoate, 4-(4'- ethoxy phenyl azo)phenyl hexanoate, azoxydianisole, and cholesteryl 4-(phenyl azo)benzoate and the stabilizers comprise from 1 to 6 weight percent of thecomposition.

3. The composition of matter of claim 2 wherein the liquid crystallinematerial is selected from the group consisting of oleyl cholesterylcarbonate, cholesteryl nonanoate, cholesteryl chloride and mixturesthereof.

4. The composition of matter of claim 2 in homogeneous mixture with aprotective polymeric material selected from the group consisting ofsilicone resin, polyisoprene poly y benzyl L-glutamate, polyvinylchloride, polystyrene, polycarbonate, linear polyurethane, polyacrylateand polyester.

5. The composition of matter of claim 4 wherein the protective polymericmaterial comprises between about 1 to 20 weight percent of thehomogeneous mixture.

6. A detector element comprising a homogeneous mixture of a clearprotective polymeric plastic material and a sensor compositioncomprising cholesteric liquid crystalline material and an ultravioletradiation stabilizer selected from the group consisting of aromatic azocompounds and aromatic azoxy compounds.

7. The detector element of claim 6 wherein the protective polymericmaterial is selected from the group consisting of silicone resin,polyisoprene, poly 'y -benzyl L-glutamate, polyvinyl chloride,polystyrene, polycarbonate, linear polyurethane, polyacrylate andpolyester.

8. The detector element of claim 7 wherein the sensor compositioncontains cholesteric liquid crystalline material and from 1 to 6 weightpercent of an ultraviolet radiation stabilizer selected from the groupconsisting of 4-(phenyl azo) phenol, 4-(4'-ethoxy phenyl azo) phenylIO-undecenoate, 4-(4'- ethoxy phenyl azo) phenyl hexanoate,azoxydianisole, and cholesteryl 4-(phenyl azo) benzoate.

oleyl cholesteryl carbonate, cholesteryl nonanoate, cholesteryl chlorideand mixtures thereof.

11. A method of making a detector element article comprising the steps:

1. adding from about 0.3 to 7 weight percent of an ultraviolet radiationstabilizer selected from the group consisting of aromatic azo compoundsand aromatic azoxy compounds to a cholesteric liquid crystallinematerial to form a stabilizer liquid crystal material,

2. mixing and dissolving the stabilizer-liquid crystal material in asolvent to form a first solution, and thereafter 3. homogeneously mixingthe stabilizer-liquid crystalline solution with a liquid comprising aclear protective polymeric material selected from the group consistingof silicone resin, polyisoprene, poly y -benzyl L-glutamate, polyvinylchloride, polystyrene, polycarbonate, linear polyurethane, andpolyester, said mixture being deposited on a substrate and the solventevaporated to form a homogeneously protectively packagedstabilizer-liquid crystalline detecting article.

12. The method of claim 11 wherein the stabilizer is selected from thegroup consisting of 4-(phenyl azo) phenol, 4-(4'-ethoxy phenyl azo)phenyl IO-undecenoate, 4-(4'- ethoxy phenyl azo) phenyl hexanoate,azoxydianisole, and cholesteryl 4-(phenyl azo) benzoate.

2. The composition of matter of claim 1 wherein the stabilizers areselected from the group consisting of 4-(phenyl azo) phenol,4-(4''-ethoxy phenyl azo) 10-undecenoate, 4-(4''-ethoxy phenyl azo)phenyl hexanoate, azoxydianisole, and cholesteryl 4-(phenyl azo)benzoate and the stabilizers comprise from 1 to 6 weight percent of thecomposition.
 2. mixing and dissolving the stabilizer-liquid crystalmaterial in a solvent to form a first solution, and thereafter 3.homogeneously mixing the stabilizer-liquid crystalline solution with aliquid comprising a clear protective polymeric material selected fromthe group consisting of silicone resin, polyisoprene, poly gamma -benzylL-glutamate, polyvinyl chloride, polystyrene, polycarbonate, linearpolyurethane, and polyester, said mixture being deposited on a substrateand the solvent evaporated to form a homogeneously protectively packagedstabilizer-liquid crystalline detecting article.
 3. The composition ofmatter of claim 2 wherein the liquid crystalline material is selectedfrom the group consisting of oleyl cholesteryl carbonate, cholesterylnonanoate, cholesteryl chloride and mixtures thereof.
 4. The compositionof matter of claim 2 in homogeneous mixture with a protective polymericmaterial selected from the group consisting of silicone resin,polyisoprene poly gamma -benzyl L-glutamate, polyvinyl chloride,polystyrene, polycarbonate, linear polyurethane, polyacrylate andpolyester.
 5. The composition of matter of claim 4 wherein theprotective polymeric material comprises between about 1 to 20 weightpercent of the homogeneous mixture.
 6. A detector element comprising ahomogeneous mixture of a clear protective polymeric plastic material anda sensor composition comprising cholesteric liquid crystalline materialand an ultraviolet radiation stabilizer selected from the groupconsisting of aromatic azo compounds and aromatic azoxy compounds. 7.The detector element of claim 6 wherein the protective polymericmaterial is selected from the group consisting of silicone resin,polyisoprene, poly gamma -benzyl L-glutamate, polyvinyl chloride,polystyrene, polycarbonate, linear polyurethane, polyacrylate andpolyester.
 8. The detector element of claim 7 wherein the sensorcomposition contains cholesteric liquid crystalline material and from 1to 6 weight percent of an ultraviolet radiation stabilizer selected fromthe group consisting of 4-(phenyl azo) phenol, 4-(4''-ethoxy phenyl azo)phenyl 10-undecenoate, 4-(4''-ethoxy phenyl azo) phenyl hexanoate,azoxydianisole, and cholesteryl 4-(phenyl azo) benzoate.
 9. The detectorelement of claim 7 wherein the sensor composition contains cholestericliquid crystalline material and from about 0.3 to 7 weight percent of anultraviolet radiation stabilizer selected from the group consisting ofsubstituted aromatic azo compounds and substituted aromatic azoxycompounds wherein the substituents on the aryl rings of the azo andazoxy compounds are selected from the group consisting of alkyl, halo,nitro, hydroxy, alkoxy, aroxy, ether and cyano substitutions.
 10. Thedetector element of claim 8 wherein the liquid crystalline material isselected from the group consisting of oleyl cholesteryl carbonate,cholesteryl nonanoate, cholesteryl chloride and mixtures thereof.
 11. Amethod of making a detector element article comprising the steps: 12.The method of claim 11 wherein the stabilizer is selected from the groupconsisting of 4-(phenyl azo) phenol, 4-(4''-ethoxy phenyl azo) phenyl10-undecenoate, 4-(4''-ethoxy phenyl azo) phenyl hexanoate,azoxydianisole, and cholesteryl 4-(phenyl azo) benzoate.